Light-emitting diodes (LEDs) can be used in retro-fitting, i.e. replacing traditional light sources such as halogen, fluorescent, or incandescent light sources. The advantages of a retro-fit luminaire are that the lamp design remains essentially unchanged, and a significant reduction in power consumption is achieved. In order to use an LED light source (typically one or more strings of LEDs) in a “traditional” lamp, it must be adapted to existing design elements such as socket design, housing shape, etc. Furthermore, the traditional lamps were developed for different power supplies—incandescent lamps can be used with a mains power supply, while a halogen lamp generally requires a transformer to lower the supply voltage. A lighting load based on semiconductor devices such as LEDs requires a highly controllable driver or power converter such as a switched-mode power supply to provide the necessary input power to the semiconductor devices. The power converter is configured to meet the requirements of the lighting load. However, the developing, testing and manufacturing of a dedicated driver for each possible application—and for each possible power level in an application—is prohibitively cost-intensive. Furthermore, in order to be able to use LEDs to replace a certain type of traditional light source, i.e. to retrofit a traditional lamp with LEDs, the driver for the LEDs must be realized to fit into the existing housing and must be able to deal with the power supply for which that lamp was originally designed. Since the physical size of a driver such as a switched-mode power supply generally increases with increased power rating, lamps with different power ratings might require different-sized housings for their drivers. For example, the magnetic component (coil or transformer) of a switched-mode powers supply is typically quite large, and an increase in power level means an increase in the size of the magnetic component. These may not fit into the available space in certain lighting applications. The requirement to provide larger lamp housings to accommodate larger components significantly adds to the overall design cost of such lighting applications.
EP1691583 A1 discloses a LED driver for a lighting load. The lighting load comprises three single LEDs. The driver comprises a switch such that just one LED of the three LEDs is driven at a time. The driver comprises a switch mode driver and a linear mode driver and the two drivers provide in conjunction a single driver signal.
Therefore, it is an object of the invention to provide an improved driver design that overcomes the problems outlined above.